The hypothesis that magnetic fields influence breast cancer risk is based on the existence of a plausible mechanism related to the effect of magnetic fields on melatonin along with some limited epidemiologic data. The study has two following specific goals. 1. To determine if residential exposure to magnetic fields, as assessed by wiring configuration coding and direct measurements is associated with the risk of breast cancer. 2. A secondary hypothesis is that particular combinations of the alternating current (AC) magnetic field and the direct current (DC) magnetic field, increase the risk of breast cancer. This hypothesis was prompted by observations of biologic effects at particular combinations of the AC and DC fields in several experimental systems. The study is a case-control study nested within a cohort study of risk factors for breast and other cancers among approximately 54,000 African-American and Latino women in Los Angeles County. A technician visits each case and control to make measurements and administer an interview on residential history and sources of exposure to magnetic fields and light at night, an effector of melatonin. All cases of breast cancer diagnosed over the 5 year study period will be enrolled -- we estimate 500-600 cases. An equal number of controls will be enrolled. We are also interested in extending this research to examine the plausibility to the proposed mechanism based on the ability of magnetic fields to suppress melatonin.